Polymeric films are widely used in the packaging field for the packaging of products, especially pharmaceutical or food products. Films used for the packaging of food generally contain multiple layers, in which each layer adds certain desired physical or chemical properties to the completed film. For example, a “barrier” layer may serve to protect the packaged product from physical stresses caused by the normal handling of the product during packaging, shipping, or during commercialization. In addition, a heat sealant layer may be utilized to bond films together to form packages for products, such as pharmaceutical or food products. Further, the combination of layers may allow for a film that has favorable physical properties, such as, for example, strength, stiffness, abrasion and chemical resistance.
One such layer that may be utilized as a barrier layer and to increase the toughness of films made therefrom is a layer comprising polyamide. Polyamide is a high molecular weight polymer having amide linkages along the molecular chain structure. Nylon polyamides, which are synthetic polyamides, have favorable physical properties of high strength, stiffness abrasion and chemical resistance.
Aggressive products, for example, moisture-containing pharmaceutical products, such as liniment, are often packaged in a polyamide, such as nylon, film as the outer layer of a package, which offers a dimensionally stable print surface as well as barrier and flex-crack resistance. The combination of a polymeric coating applied to a nylon film, for example, a biaxially oriented nylon (BON) film, results in a structure that is softer and more formable than typical film laminations.
It is, of course, generally known to provide multilayer polymeric films that may have high strength, stiffness, abrasion resistance and/or chemical. Many such films may have a barrier layer of ethylene-vinyl alcohol copolymer (EVOH) sandwiched by layers comprising a nylon polyamide. In some cases, these layers do not gel together, so a multilayer film would delaminate. To overcome this issue, small layers of special adhesive resins, which are generally known as tie layers, are used in between the nylon and EVOH. For example, a five-layer film comprises five different layers of film such as Nylon//Tie//EVOH//Tie//Nylon.
U.S. Pat. No. 6,068,933 discloses a clear multilayer polymeric film of a structure, including a core layer of ethylene-vinyl alcohol copolymer (EVOH) sandwiched between two nylon layers, or two nylon inner layers and a nylon outer layer, represents an improved multilayer thermoformable polymeric layer. U.S. Pat. No. 4,615,926 discloses a multiple layer packaging film, packages made therefrom and a method of opening the packages. The packaging film has the following layer structure:
Ionomer//tie layer//ethylene vinyl alcohol (EVOH)//adhesive layer//polyethylene layer//tie layer//polypropylene.
U.S. Pat. No. 6,902,817 discloses a permeable membrane for volatile substances, such as a room air fragrance and other aromatic products, that satisfies the need for a permeable membrane which provides greater controlled release and increased permeation for a broad range of fragrances. US Patent Application 2010/0272869 discloses a coextruded multilayer film comprising a core layer including an ethylene vinyl alcohol copolymer; two polyamide layers; a first outer layer; a second outer layer; and two tie layers each adhering a polyamide layer to a respective outer layer. U.S. Pat. No. 4,729,926 discloses a polymeric film having the following structure:
Tie layer//nylon layer//ethylene-vinyl alcohol copolymer (EVOH) layer//nylon layer//low density polyethylene layer//primer layer//outer layer.
Unfortunately, mottling can occur when such nylon is used in packages for moisture containing products, for example, liniment. Moisture from the products diffuses through the package into the nylon layer. The nylon absorbs the water, creating the mottling and swelling in size to create a gap between the package and the products. Such mottling may appear over time, such as after the package is shipped for sale. Accordingly, absorption of water from moisture containing products results in a poor package appearance and increased potential for product mold, because mold is better able to grow when there is air in contact with a moisture containing product. Indeed, the tightness of a package around a moisture containing product may be more critical for preventing mold growth than the extent of the oxygen barrier characteristics of a package.